1. Field of the Invention:
The present invention relates in general to a driving power transmission for transmitting rotational power from an input shaft to an output shaft and vice versa at the occurrence of the rotational speed difference between the input and output shafts and more particularly, to such a driving power transmission as particularly designed for incorporation in motor vehicles.
2. Discussion of the Prior Art:
As disclosed in U.S. Pat. No. 4,719,998 for example, there has been known a four-wheel drive vehicle of the type that a conventional oil pump is driven upon occurrence of a rotational speed difference between two coaxial drive shafts respectively connected to front and rear wheels and that a hydraulically operated clutch for effecting selective driving connection between the two drive shafts is operated in response to the discharge pressure from the oil pump corresponding to the rotational speed difference.
In a torque transmission device for the known four-wheel drive vehicle, the oil pump of plunger or vane type is interposed between the front and rear wheel drive shafts so as to discharge oil of the pressure corresponding to the rotational speed difference between the drive shafts. The pressurized oil from the pump is admitted into a cylinder chamber of the hydraulic clutch through a fluid passage formed in one of the drive shafts, so that the clutch can be hydraulically operated to drivingly connect the drive shafts with each other.
However, in the torque transmission device of the aforementioned type, the conventional oil pump interconnected between the front and rear drive shafts is relatively large in axial width thereof, and the fluid passage has to be provided for admitting the discharge pressure from the oil pump into the hydraulic clutch. This disadvantageously results in a complicated and large construction of the torque transmission device as well as in an increased cost for manufacturing.